What would you do with the water that you displaced by putting in the water? The thing that could potentially work is to pump the water from the pool, into a hot water heater, and then back to the pool, but I am sure the hot water heater needs the water to be in there a while before it will heat it up. I have heard of other methods to heat a pool. A coil of black plastic pipe, pump the water through it and set it in the sun back into the pool. Or else a coil of copper pipe, like 1", and set it in a fire and pump it back in to the pool. The copper one I get a feeling is pure BS, but the plastic one could work, but slowly I would say.

I got a pool heater off of ebay for 560 dollars I believe, and I put it in myself, it is one of the best investments you could make if you ask me. I raised the temp of the water over 20 degrees in like 8 hours. I have had the water up to 96 before, which happens to be too hot for my tastes, but it is awesome.